The present invention relates to an optical data recording media in which optical characteristics of a material vary dependent on differences in the heat history or processes of temperature elevation due to laser beam irradiation and subsequent cooling. In addition, the present invention relates to a method for recording, erasing and reproducing an optical data on the recording media.
Optical disks for recording and reproducing data using laser beams have drawn great attention as portable file memory means having a large storage capacity, and various kinds of optical disks such as of read-only optical disks, write-once optical disks and overwritable optical disks are known. As overwritable types of the disks, phase-change optical disks, exchange-coupled magneto-optical disks and magnetic field modulation magneto-optical disks are known. In these overwritable optical disks, the phase-change optical disk is adapted to record or erase data by changing the optical characteristics caused by the difference of the heat history or process of temperature elevation due to laser beam irradiation and subsequent cooling. In such a phase-change optical disk, data is recorded, or the latest data is written over the previously recorded data, by modulating the energy of the irradiating light beam between two intensity levels respectively corresponding to optically discriminating two states. Here, there are two energy levels of the irradiating light, that is, the higher level and the lower level, and the former corresponds to "recording power" and the latter corresponds to "erasing power". The reading of data is usually effected by detecting variations in transmittance or reflectance between the aforementioned two states.
Meanwhile, most of these optical data recording media tend to be formed on a transparent substrate provided with guide grooves. As shown in FIG. 1, V-shaped guide grooves 6 are formed on a surface of a substrate 1, and wide convex portions 7 are disposed between guide grooves 6. Accordingly, grooves and convex portions are also formed on a recording layer 3 formed on substrate 1. Hereinafter, for convenience, a substrate having V-shaped guide grooves 6 as shown in FIG. 1 will be referred to as "V-groove substrate."
In a phase-change type optical disk, reproduction signals are regenerated by detecting variations in the reflectance or transmittance consequent upon the physical phase changes. In such a case, the amplitude of reproduction signals becomes larger as the size of recorded marks becomes greater. And the amplitude of the reproduction signals saturates when the size of the recorded marks is equivalent to, or larger than, the beam size of a laser beam spot 10 used for reproduction. For this reason, sufficient amplitude of reproduction signals cannot be obtained unless a recording mark is large as compared to the beam size of the reproduction laser. Nevertheless, since recording marks in practice are small compared to the beam size of the reproduction laser, signals obtained cannot always exhibit sufficient amplitude. Such deterioration of the amplitude of the reproduction signals is more notable as the size of the recording mark becomes smaller because of the recording frequency becomes higher. This also makes it difficult to improve the recording density of phase-change type optical disks.
In order to obtain reproduction signals having a large amplitude, it is possible to optimally design the media structure such that the change in reflectance due to the phase transition may be enhanced. But this also increases the change of the absorptance and causes another problem. That is, upon overwriting operation, signals to be overwritten are apt to be modulated under affection of the prior signals or data, so that the performance of the overwriting operation is degraded.
On the other hand, in order to improve the recording density, Japanese Patent Laid-Open Application (KOKAI) Nos. 2-73537 and 2-113451 disclose methods of providing reproduction signals having sufficient amplitude even for small recording marks. That is, in these methods, the layers structure of the recording media is determined so that the change in reflectance may be small and an optical phase change or difference of the reflected light may be nearly equal to (1.+-.2n).pi.. Such a material having small change in reflectance but causing a large phase difference, however, has a strong dependence of its optical characteristics upon the thickness of each layer of the media, and this requires that the thickness of the layers be within very tight tolerances.